Method for producing esters of triazolopyrimidine derivatives free of enantiomers using vinylesters with lipase

ABSTRACT

A process for preparing enantiomerically pure esters of the formula I (Ia and Ib)                    
     where the substituents have the following meanings: 
     R 1    
     hydrogen or substituted or unsubstituted C 1 -C 6 -alkyl, C 1 -C 6 -alkoxy or Cl-C 6 -alkanoyl, 
     R 2  and R 3    
     independently of one another hydrogen or substituted or unsubstituted C 1 -C 6 -alkyl, C 1 -C 6 -alkoxy, C 1 -C 6 -alkanoyl, C 1 -C 6 -alkylthio, C 1 -C 6 -alkylsulfinyl or C 1 -C 6 -alkylsulfonyl, 
     R 4  and R 5    
     R 4 ≠R 5  and independently of one another hydrogen or substituted or unsubstituted C 1 -C 6 -alkyl or R 4  and R 5  form together with the carbon atoms to which they are bonded a substituted or unsubstituted C 3 -C 6 -cycloalkylidene, 
     R 6    
     substituted or unsubstituted aryl, C 1 -C 20 -alkyl, C 3 -C 20 -alkenyl, C 3 -C 20 -alkynyl, C 1 -C 20 -alkoxy-C 1 -C 20 -alkyl 
     comprises converting racemic compounds of the formula II,                    
     where the substituents R 1  to R 5  have the abovementioned meanings, with a lipase or esterase in the presence of vinyl esters of the formula III,                    
     where R 6  has the abovementioned meaning, and R 7  is hydrogen or methyl, into compounds of the formula I.

BACKGROUND OF THE INVENTION

The invention relates to a process for preparing enantiomerically purealcohols.

Kinetic resolutions of racemic esters with lipases and esterases aredescribed in a large number of publications and patents. Only a fewstudies on the resolution of racemic esters or alcohols which have aheteroaromatic radical have been published.

Thus, for example, Akita et al. (Tetrahedron Lett. 27 (1986), No. 43,5241-5244) describe the enantioselective hydrolysis of methyl3-acetoxy-3-(2-furyl)-2-methylpropanoates or methyl3-acetoxy-3-(2-thienyl)-2-methylpropanoates with an Aspergillus nigerlipase.

De Amici et al. describe, in J. Org. Chem. 54 (1989) 2646-2650, anenzymatically catalyzed transesterification with porcine liver esterase,Candida cylindracea lipase, chymotrypsin, subtilisin, porcine pancreaticlipase and lipase P.

Tsukamoto et al. (Tetrahedron Asym. 2 (1991), No. 8,759-762) describethe synthesis of (R)- and(S)-N,N-diethyl-2,2-difluoro-3-(2-furyl)-3-hydroxypropionamide from thecorresponding esters with Candida cylindracea lipase MY and P in water.

DE/OS 3743824 and Schneider et al. (Tetrahedron Asym. 3 (1992), No. 7,827-830) describe the preparation of 1-pyridylethanol.

The disadvantages of these methods are the low selectivity of theenzymes, the low enantiomeric purities of the products obtained, the lowchemical yields, and the large amounts of enzyme required for thereaction.

An optimal racemate resolution should advantageously comply with anumber of conditions, such as:

1. high enantiomeric purity of the antipodes

2. high chemical yield

3. high enzyme selectivity

4. small amounts of catalyst (amounts of enzyme)

5. good solubility of precursor and product under the reactionconditions

6. good space-time yield

7. easy purification of the products

8. low-cost synthesis.

WO 95/10521 claims 1,2,4-triazolo[1,5-a]pyrimidines, their chemicalsynthesis and their use in pharmaceutical compositions.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to develop a stereoselectivesynthesis of intermediates for 1,2,4-triazolo-[1,5-a]pyrimidines whichprovides these compounds advantageously with high optical purities andgood chemical yields and which permits easy workup of the products.

DETAILED DESCRIPTION OF THE INVENTION

We have found that this object is achieved by a process for preparingenantiomerically pure esters of the formula I (Ia or Ib)

 where the substituents have the following meanings:

 R¹

 hydrogen or substituted or unsubstituted C₁-C₆-alkyl, C₃-C₆-alkoxy orC₁-C₆-alkanoyl,

 R² and R³

 independently of one another hydrogen or substituted or unsubstitutedC₁-C₆-alkyl, C₁-C₆-alkoxy, C₁-C₆-alkanoyl, C₁-C₆-alkylthio,C₁-C₆-alkylsulfinyl or C₁-C₆-alkylsulfonyl,

 R⁴ and R⁵

 R⁴≠R⁵ and independently of one another hydrogen or substituted orunsubstituted C₁-C₆-alkyl or R⁴ and R⁵ form together with the carbonatoms to which they are bonded a substituted or unsubstitutedC₃-C₆-cycloalkylidene,

 R⁶

 substituted or unsubstituted aryl, C₁-C₂₀-alkyl, C₃-C₂₀-alkenyl,C₃-C₂₀-alkynyl, C₁-C₂₀-alkoxy-C₁-C₂₀-alkyl

 which comprises converting racemic compounds of the formula II,

 where the substituents R¹ to R⁵ have the abovementioned meanings, witha lipase or esterase in the presence of vinyl esters of the formula III,

 where R⁶ has the abovementioned meaning, and R⁷ is hydrogen or methyl,into compounds of the formula I.

R¹ in the formulae I and II is hydrogen or substituted or unsubstitutedC₁-C₆-alkyl, C₁-C₆-alkoxy or C₁-C6-alkanoyl.

Examples of meanings for the radicals mentioned for R¹ are thefollowing:

 alkyl branched or unbranched Cl-C₆-alkyl chains such as methyl, ethyl,n-propyl, 1-methylethyl, n-butyl, 1-methylpropyl, 2-methylpropyl,1,1-dimethylethyl, n-pentyl, 1-methylbutyl, 2-methylbutyl,3-methylbutyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl,2,2-dimethylpropyl, 1-ethylpropyl, n-hexyl, 1-methylpentyl,2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1,1-dimethylbutyl,1,2-dimethylbutyl, 1,3-dimethylbutyl, 2,2-dimethylbutyl,2,3-dimethylbutyl, 3,3-dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl,1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl, 1-ethyl-1-methylpropyl or1-ethyl-2-methylpropyl,

 alkoxy branched or unbranched C₁-C₆-alkoxy chains as mentioned above,eg. methoxy, ethoxy, propoxy, 1-methylethoxy, butoxy, 1-methylpropoxy,2-methylpropoxy, 1,1-dimethylethoxy, pentoxy, 1-methylbutoxy,2-methylbutoxy, 3-methylbutoxy, 1,1-dimethylpropoxy,1,2-dimethylpropoxy, 2,2-dimethylpropoxy, 1-ethylpropoxy, hexoxy,1-methylpentoxy, 2-methylpentoxy, 3-methylpentoxy, 4-methylpentoxy,1,1-dimethylbutoxy, 1,2-dimethylbutoxy, 1,3-dimethylbutoxy,2,2-dimethylbutoxy, 2,3-dimethylbutoxy, 3,3-dimethylbutoxy,1-ethylbutoxy, 2-ethylbutoxy, 1,1,2-trimethylpropoxy,1,2,2-trimethylpropoxy, 1-ethyl-l-methylpropoxy or1-ethyl-2-methylpropoxy,

 alkanoyl branched or unbranched C₁-C₆-alkanoyl chains such asmethanoyl, ethanoyl, propanoyl, 1-methylethanoyl, butanoyl,1-methylpropanoyl, 2-methylpropanoyl, 1,1-dimethylethanoyl, pentanoyl,1-methylbutanoyl, 2-methylbutanoyl, 3-methylbutanoyl,1,1-dimethylpropanoyl, 1,2-dimethylpropanoyl, 2,2-dimethylpropanoyl,1-ethylpropanoyl, hexanoyl, 1-methylpentanoyl, 1,2-methylpentanoyl,3-methylpentanoyl, 4-methylpentanoyl, 1,1-dimethylbutanoyl,1,2-dimethylbutanoyl, 1,3-dimethylbutanoyl, 2,2-dimethylbutanoyl,2,3-dimethylbutanoyl, 3,3-dimethylbutanoyl, 1-ethylbutanoyl,2-ethylbutanoyl, 1,1,2-trimethylpropanoyl, 1,2,2-trimethylpropanoyl,1-ethyl-1-methylpropanoyl and 1-ethyl-2-methylpropanoyl.

Suitable substituents for the alkyl, alkoxy or alkanoyl radicalsmentioned for R¹ are one or more substituents such as halogen such asfluorine, chlorine, bromine, cyano, nitro, amino, mercapto, alkyl,alkoxy or aryl.

R² and R³ in the formulae I and II are, independently of one another,hydrogen or substituted or unsubstituted C₁-C₆-alkyl, C₁-C₆-alkoxy,C₁-C₆-alkanoyl, C₁-C₆-alkylthio, C₁-C₆-alkylsulfinyl orC₁-C₆-alkylsulfonyl.

Examples of meanings of the radicals mentioned for R² and R³ are thefollowing:

 alkyl branched or unbranched C₁-C₆-alkyl chains such as methyl, ethyl,n-propyl, 1-methylethyl, n-butyl, 1-methylpropyl, 2-methylpropyl,1,1-dimethylethyl, n-pentyl, 1-methylbutyl, 2-methylbutyl,3-methylbutyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl,2,2-dimethylpropyl, 1-ethylpropyl, n-hexyl, 1-methylpentyl,2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1,1-dimethylbutyl,1,2-dimethylbutyl, 1,3-dimethylbutyl, 2,2-dimethylbutyl,2,3-dimethylbutyl, 3,3-dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl,1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl, 1-ethyl-l-methylpropyl or1-ethyl-2-methylpropyl,

 alkoxy branched or unbranched C₁-C₆-alkoxy chains as mentioned above,eg. methoxy, ethoxy, propoxy, 1-methylethoxy, butoxy, 1-methylpropoxy,2-methylpropoxy, 1,1-dimethylethoxy, pentoxy, 1-methylbutoxy,2-methylbutoxy, 3-methylbutoxy, 1,1-dimethylpropoxy,1,2-dimethylpropoxy, 2,2-dimethylpropoxy, 1-ethylpropoxy, hexoxy,1-methylpentoxy, 2-methylpentoxy, 3-methylpentoxy, 4-methylpentoxy,1,1-dimethylbutoxy, 1,2-dimethylbutoxy, 1,3-dimethylbutoxy,2,2-dimethylbutoxy, 2,3-dimethylbutoxy, 3,3-dimethylbutoxy,1-ethylbutoxy, 2-ethylbutoxy, 1,1,2-trimethylpropoxy,1,2,2-trimethylpropoxy, 1-ethyl-l-methylpropoxy or1-ethyl-2-methylpropoxy,

 alkanoyl branched or unbranched C₁-C₆-alkanoyl chains such asmethanoyl, ethanoyl, propanoyl, 1-methylethanoyl, butanoyl,1-methylpropanoyl, 2-methylpropanoyl, 1,1-dimethylethanoyl, pentanoyl,1-methylbutanoyl, 2-methylbutanoyl, 3-methylbutanoyl,1,1-dimethylpropanoyl, 1,2-dimethylpropanoyl, 2,2-dimethylpropanoyl,1-ethylpropanoyl, hexanoyl, 1-methylpentanoyl, 1,2-methylpentanoyl,3-methylpentanoyl, 4-methylpentanoyl, 1,1-dimethylbutanoyl,1,2-dimethylbutanoyl, 1,3-dimethylbutanoyl, 2,2-dimethylbutanoyl,2,3-dimethylbutanoyl, 3,3-dimethylbutanoyl, 1-ethylbutanoyl,2-ethylbutanoyl, 1,1,2-trimethylpropanoyl, 1,2,2-trimethylpropanoyl,1-ethyl-l-methylpropanoyl and 1-ethyl-2-methylpropanoyl,

 alkylthio branched or unbranched C₁-C₆-alkylthio chains such asmethylthio, ethylthio, n-propylthio, 1-methylethylthio, n-butylthio,1-methylpropylthio, 2-methylpropylthio, 1,1-dimethylethylthio,n-pentylthio, 1-methylbutylthio, 2-methylbutylthio, 3-methylbutylthio,2,2-dimethylpropylthio, 1-ethylpropylthio, n-hexylthio,1,1-dimethylpropylthio, 1,2-dimethylpropylthio, 1-methylpentylthio,2-methylpentylthio, 3-methylpentylthio, 4-methylpentylthio,1,1-dimethylbutylthio, 1,2-dimethylbutylthio, 1,3-dimethylbutylthio,2,2-dimethylbutylthio, 2,3-dimethylbutylthio, 3,3-dimethylbutylthio,1-ethylbutylthio, 2-ethylbutylthio, 1,1,2-trimethylpropylthio,1,2,2-trimethylpropylthio, 1-ethyl-1-methylpropylthio or1-ethyl-2-methylpropylthio,

 alkylsulfinyl branched or unbranched C₁-C₆-alkylsulfinyl chains such asmethylsulfinyl, ethylsulfinyl, n-propylsulfinyl, 1-methylethylsulfinyl,n-butylsulfinyl, 1-methylpropylsulfinyl, 2-methylpropylsulfinyl,1,1-dimethylethylsulfinyl, n-pentylsulfinyl, 1-methylbutylsulfinyl,2-methylbutylsulfinyl, 3-methylbutylsulfinyl,1,1-dimethylpropylsulfinyl, 1,2-dimethylpropylsulfinyl,2,2-dimethylpropylsulfinyl, 1-ethylpropylsulfinyl, n-hexylsulfinyl,1-methylpentylsulfinyl, 2-methylpentylsulfinyl, 3-methylpentylsulfinyl,4-methylpentylsulfinyl, 1,1-dimethylbutylsulfinyl,1,2-dimethylbutylsulfinyl, 1,3-dimethylbutylsulfinyl,2,2-dimethylbutylsulfinyl, 2,3-dimethylbutylsulfinyl,3,3-dimethylbutylsulfinyl, 1-ethylbutylsulfinyl, 2-ethylbutylsulfinyl,1,1,2-trimethylpropylsulfinyl, 1,2,2-trimethylpropylsulfinyl,1-ethyl-1-methylpropylsulfinyl and 1-ethyl-2-methylpropylsulfinyl,

 alkylsulfonyl branched or unbranched C₁-C₆-alkylsulfonyl chains such asmethylsulfonyl, ethylsulfonyl, n-propylsulfonyl, 1-methylethylsulfonyl,n-butylsulfonyl, 1-methylpropylsulfonyl, 2-methylpropylsulfonyl,1,1-dimethylethylsulfonyl, n-pentylsulfonyl, 1-methylbutylsulfonyl,2-methylbutylsulfonyl, 3-methylbutylsulfonyl,1,1-dimethylpropylsulfonyl, 1,2-dimethylpropylsulfonyl,2,2-dimethylpropylsulfonyl, 1-ethylpropylsulfonyl, n-hexylsulfonyl,1-methylpentylsulfonyl, 2-methylpentylsulfonyl, 3-methylpentylsulfonyl,4-methylpentylsulfonyl, 1,l-dimethylbutylsulfonyl,1,2-dimethylbutylsulfonyl, 1,3-dimethylbutylsulfonyl,2,2-dimethylbutylsulfonyl, 2,3-dimethylbutylsulfonyl,3,3-dimethylbutylsulfonyl, 1-ethylbutylsulfonyl, 2-ethylbutylsulfonyl,1,1,2-trimethylpropylsulfonyl, 1,2,2-trimethylpropylsulfonyl,1-ethyl-1-methylpropylsulfonyl and 1-ethyl-2-methylpropylsulfonyl.

Suitable substituents for the alkyl, alkoxy, alkanoyl, alkylthio,alkylsulfinyl or alkylsulfonyl radicals mentioned for R² and R³ are oneor more substituents such as halogen such as fluorine, chlorine,bromine, cyano, nitro, amino, mercapto, alkyl, alkoxy or aryl.

R⁴ and R⁵ are not the same and in the formulae I and II are,independently of one another, hydrogen or substituted or unsubstitutedC₁-C₆-alkyl or R⁴ and R⁵ form together with the carbon atoms to whichthey are bonded a substituted or unsubstituted C₃-C₆-cycloalkylidene.

Examples of meanings of the radicals mentioned for R⁴ and R⁵ are thefollowing:

 alkyl branched or unbranched C₁-C₆-alkyl chains such as methyl, ethyl,n-propyl, 1-methylethyl, n-butyl, 1-methylpropyl, 2-methylpropyl,1,1-dimethylethyl, n-pentyl, 1-methylbutyl, 2-methylbutyl,3-methylbutyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl,2,2-dimethylpropyl, 1-ethylpropyl, n-hexyl, 1-methylpentyl,2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1,1-dimethylbutyl,1,2-dimethylbutyl, 1,3-dimethylbutyl, 2,2-dimethylbutyl,2,3-dimethylbutyl, 3,3-dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl,1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl, 1-ethyl-l-methylpropyl orl-ethyl-2-methylpropyl,

 cycloalkylidene branched or unbranched C₃-C₆-cycloalkylidene chainssuch as cyclopropylidene, ethylcyclopropylidene,dimethylcyclopropylidene, methylethylcyclopropylidene, cyclobutylidene,ethylcyclobutylidene, dimethylcyclobutylidene, cyclopentylidene ormethylcyclopentylidene.

Suitable substituents for the alkyl or cycloalkylidene radicalsmentioned for R⁴ and R⁵ are one or more substituents such as halogensuch as fluorine, chlorine, bromine, cyano, nitro, amino, mercapto,alkyl, alkoxy or aryl.

R⁶ in the formulae I and III is substituted or unsubstituted aryl,C₁-C₂₀-alkyl, C₁-C₂₀-alkenyl, C₁-C₂₀-alkynyl orC₁-C₂₀-alkoxy-C₁-C₂₀-alkyl.

Examples of meanings for the radicals mentioned for R⁶ are thefollowing:

 aryl simple or fused aromatic ring systems which are unsubstituted orsubstituted by one or more radicals such as halogen such as fluorine,chlorine or bromine, cyano, nitro, amino, mercapto, alkyl, alkoxy orother saturated or unsaturated nonaromatic rings or ring systems, or areunsubstituted or substituted by at least one other C₁--C₁₀-alkyl chain,or are linked via a C₁-C₁₀-alkyl chain to the basic framework, andphenyl and naphthyl are preferred as aryl radical,

 alkyl branched or unbranched C₁-C₂₀-alkyl chains such as methyl, ethyl,n-propyl, 1-methylethyl, n-butyl, 1-methylpropyl, 2-methylpropyl,1,1-dimethylethyl, n-pentyl, 1-methylbutyl, 2-methylbutyl,3-methylbutyl, 2,2-dimethylpropyl, 1-ethylpropyl, n-hexyl,1,1-dimethylpropyl, 1,2-dimethylpropyl, 1-methylpentyl, 2-methylpentyl,3-methylpentyl, 4-methylpentyl, 1,1-dimethylbutyl, 1,2-dimethylbutyl,1,3-dimethylbutyl, 2,2-dimethylbutyl, 2,3-dimethylbutyl,3,3-dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl, 1,1,2-trimethylpropyl,1,2,2-trimethylpropyl, 1-ethyl-1-methylpropyl, 1-ethyl-2-methylpropyl,n-heptyl, n-octyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl, n-tetradecyl,n-hexadecyl, n-octadecyl or n-eicosyl, and Cl-C₈-alkyl chains arepreferred, C₂-C₄-alkyl chains are particularly preferred and substitutedC₂-C₄-alkyl chains are very particularly preferred (see below forsubstituents), such as chloroethyl or methoxyethyl,

 alkenyl branched or unbranched C₃-C₂₀-alkenyl chains such as propenyl,1-butenyl, 2-butenyl, 3-butenyl, 2-methylpropenyl, 1-pentenyl,2-pentenyl, 3-pentenyl, 4-pentenyl, 1-methyl-1-butenyl,2-methyl-l-butenyl, 3-methyl-1-butenyl, 1-methyl-2-butenyl,2-methyl-2-butenyl, 3-methyl-2-butenyl, 1-methyl-3-butenyl,2-methyl-3-butenyl, 3-methyl-3-butenyl, 1,1-dimethyl-2-propenyl,1,2-dimethyl-1-propenyl, 1,2-dimethyl-2-propenyl, 1-ethyl-l-propenyl,1-ethyl-2-propenyl, 1-hexenyl, 2-hexenyl, 3-hexenyl, 4-hexenyl,5-hexenyl, 1-methyl-1-pentenyl, 2-methyl-b 1-pentenyl,3-methyl-1-pentenyl, 4-methyl-1-pentenyl, 1-methyl-2-pentenyl,2-methyl-2-pentenyl, 3-methyl-2-pentenyl, 4-methyl-2-pentenyl,1-methyl-3-pentenyl, 2-methyl-3-pentenyl, 3-methyl-3-pentenyl,4-methyl-3-pentenyl, 1-methyl-4-pentenyl, 2-methyl-4-pentenyl,3-methyl-4-pentenyl, 4-methyl-4-pentenyl, 1,1-dimethyl-2-butenyl,1,1-dimethyl-3-butenyl, 1,2-dimethyl-1-butenyl, 1,2-dimethyl-2-butenyl,1,2-dimethyl-3-butenyl, 1,3-dimethyl-1-butenyl, 1,3-dimethyl-2-butenyl,1,3-dimethyl-3-butenyl, 2,2-dimethyl-3-butenyl, 2,3-dimethyl-l-butenyl,2,3-dimethyl-2-butenyl, 2,3-dimethyl-3-butenyl, 3,3-dimethyl-l-butenyl,3,3-dimethyl-2-butenyl, 1-ethyl-1-butenyl, 1-ethyl-2-butenyl,1-ethyl-3-butenyl, 2-ethyl-1-butenyl, 2-ethyl-2-butenyl,2-ethyl-3-butenyl, 1,1,2-trimethyl-2-propenyl,1-ethyl-1-methyl-2-propenyl, 1-ethyl-2-methyl-l-propenyl,1-ethyl-2-methyl-2-propenyl, 1-heptenyl, 2-heptenyl, 3-heptenyl,4-heptenyl, 5-heptenyl, 6-heptenyl, 1-octenyl, 2-octenyl, 3-octenyl,4-octenyl, 5-octenyl, 6-octenyl or 7-octenyl, and unsaturated alkylchains which can be derived from natural fatty acids, such as mono- orpolyunsaturated C₁₆-, C₁₈- or C20-alkyl chains are preferred,

 alkynyl branched or unbranched C₃-C₂₀-alkynyl chains such asprop-1-yn-1-yl, prop-2-yn-1-yl, n-but-l-yn-1-yl, n-but-1-yn-3-yl,n-but-1-yn-4-yl, n-but-2-yn-1-yl, n-pent-1-yn-1-yl, n-pent-1-yn-3-yl,n-pent-1-yn-4-yl, n-pent-1-yn-5-yl, n-pent-2-yn-1-yl, n-pent-2-yn-4-yl,n-pent-2-yn-5-yl, 3-methyl-but-1-yn-3-yl, 3-methyl-but-1-yn-4-yl,n-hex-1-yn-1-yl, n-hex-1-yn-3-yl, n-hex-1-yn-4-yl, n-hex-1-yn-5-yl,n-hex-1-yn-6-yl, n-hex-2-yn-1-yl, n-hex-2-yn-4-yl, n-hex-2-yn-5-yl,n-hex-2-yn-6-yl, n-hex-3-yn-1-yl, n-hex-3-yn-2-yl,3-methyl-pent-1-yn-1-yl, 3-methyl-pent-1-yn-3-yl,3-methyl-pent-1-yn-4-yl, 3-methyl-pent-1-yn-5-yl,4-methyl-pent-1-yn-1-yl, 4-methyl-pent-2-yn-4-yl or4-methyl-pent-2-yn-5-yl, and C₃-C₁₀-alkynyl chains are preferred, andC₃-C₆-alkynyl chains are particularly preferred.

 alkoxyalkyl branched or unbranched C₁-C₂₀-alkoxy-C₁-C₂₀-alkyl chainssuch as methoxymethyl, methoxyethyl, methoxypropyl, ethoxymethyl,propoxymethyl, 1-methylethoxymethyl, butoxymethyl,1-methylpropoxymethyl, 2-methylpropoxymethyl, 1,1-dimethylethoxymethyl,and C₁-C₁₀-alkoxy-C₁-C₁₀-alkyl is preferred, C₁-C₆-alkoxy-C₁-C₈-alkyl isparticularly preferred and C₁-C₄-alkoxy-C₁-C₄-alkyl is very particularlypreferred. Likewise preferred are α,β-saturated alkoxyalkyl radicals.

Suitable substituents for the alkyl, alkenyl, alkynyl or alkoxyalkylradicals mentioned for R⁶ are one or more substituents such as halogensuch as fluorine, chlorine, bromine, cyano, nitro, amino, mercapto,alkyl, alkoxy or aryl.

The enzymes suitable in principle for the process according to theinvention are all lipases or esterases of nomenclature class 3.1—whichreact with ester linkages. However, lipases or esterases of microbialorigin or porcine pancreatic lipase are preferred. Examples of enzymesof microbial origin which may be mentioned are enzymes from fungi,yeasts or bacteria such as Alcaligenes sp., Achromobacter sp.,Aspergillus niger, Bacillus subtilis, Candida cylindracea, Candidalypolytica, Candida antarctica, Candida sp., Chromobacterium viscosum,Chromobacterium sp., Geotrichum candidum, Humicola lanuginosa, Mucormiehei, Penicillium camemberti, Penicillium roqueforti, Phycomycesnitens, Pseudomonas cepacia, Pseudomonas glumae, Pseudomonasfluorescens, Pseudomonas plantarii, Pseudomonas aeruginosa, Pseudomonassp., Rhizopus arrhizus, Rhizopus delemar, Rhizopus japanicus, Rhizopusniveus, Rhizopus oryzae or Rhizopus sp. Particularly preferred lipasesor esterases are those from Pseudomonas species such as Pseudomonascepacia or Pseudomonas plantarii, from Candida species such as Candidacylindracea or Candida antarctica, such as Novozym® 435 or porcinepancreatic lipase. Very particularly preferred are Pseudomonas plantariilipase, Amano P® lipase (supplied by Amano, Japan), NovozymSP523, SP524,SP525, SP526, SP539, SP435 (supplied by Novo, Denmark), Chirazyme® L1,L2, L3, L4, L5, L6, L7, L8, E1 (supplied by Boehringer Mannheim,Germany), porcine pancreatic lipase or the lipase from Pseudomonas spec.DSM 8246.

The enzymes are employed in the reaction directly or as immobilizates ona wide variety of carriers. The amount of enzyme to be added depends onthe nature of the precursor, product, the vinyl ester and the activityof the enzyme preparation. The optimal amount of enzyme for the reactioncan easily be determined by simple preliminary tests. Theenzyme/substrate ratio, calculated as molar ratio between enzyme andsubstrate, depends on the enzyme and is, as a rule, from 1:1000 to1:50000000 or more, preferably 1:100000 to 1:5000000, which means thatit is possible, for example to cleave 3 kg or more of a substrate with amolecular weight of about 100 to its enantiomers using 10 mg of anenzyme. The enantioselectivity (=E) of the enzymes is, as a rule,advantageously from 20 to 1000 in this case.

The enzymes can be used directly in the reaction as free or immobilizedenzymes or else, advantageously, after an activation step in aqueousmedium in the presence of a surface-active substance such as oleic acid,linoleic acid or linolenic acid and subsequent removal of water.

The enzyme reaction can be carried out without adding additionalsolvents or solvent mixtures only in the presence of the vinyl esters(see formula III) as solvent. It is advantageous to add other solventsor solvent mixtures to the reaction. Suitable for this in principle areall aprotic or protic solvents. All solvents inert in the reaction aresuitable, that is they must not take part in the enzyme reaction.Unsuitable examples are primary or secondary alcohols, DMF, DMSO andwater because side reactions may occur in the presence of thesesolvents—they are enzyme substrates themselves or lead to hydrolysis ofthe esters—and/or the enzymes tend to stick together and thus the enzymeactivity decreases drastically. DMF and DMSO damage enzymes in prolongedreactions, presumably due to removal of the hydrate sheath around theenzymes. Examples of suitable solvents which may be mentioned here arepure aliphatic or aromatic hydrocarbons such as hexane, cyclohexane ortoluene, halogenated hydrocarbons such as methylene chloride orchloroform, ethers such as MTBE, THF, diethyl ether, diisopropyl etheror dioxane, tertiary alcohols such as tert-butanol, tert-pentyl alcoholor propylene carbonate, ethylene carbonate or acetonitrile. It isadvantageous to have additional solvents or solvent mixtures present,particularly preferably to have toluene, diethyl ether, diisopropylether or tert-pentyl alcohol present. The solvents used for this purposeshould be as anhydrous as possible in order to prevent unspecifichydrolysis of the esters. The activity of water in the reaction canadvantageously be controlled by using molecular sieves or ammoniumsalts.

All vinyl esters are suitable in principle for the reaction, such as thevinyl esters of long-chain fatty acids (C₁₂ to C₂₀), vinylchloroacetate, vinyl acetate, vinyl propionate or vinyl butyrate, andvinyl acetate, vinyl propionate or vinyl butyrate is preferably used,and vinyl propionate or vinyl butyrate is particularly preferably used.

The reaction is advantageously carried out at from 0° C. to 75° C.,preferably from 10° C. to 60° C., particularly preferably from 15° C. to50° C.

The reaction times are from 1 to 72 hours depending on the substrate,ester and enzyme. From 1 to 3 mol of vinyl ester are added per mole ofsubstrate to be reacted.

The course of the reaction can easily be followed by conventionalmethods, for example by gas chromatography. It is sensible to stop thereaction when 50% of the racemic alcohol has reacted—maximum yield withmaximum enantiomeric purity in theory. The reaction may be stoppedearlier or later, that is before or after 50% of the racemate hasreacted, to increase the enantiomeric purity. This usually takes placeby removing the catalyst from the reaction . . . , for example byfiltering off the enzyme.

Depending on the enzyme there is selective formation of the R or S ester(see formula I, claim 1 and formulae Ia and Ib in scheme I which depictthe individual enantiomers). The other enantiomer in each case does notreact and remains unchanged at the alcohol stage (see formulae IIa andIIb in Scheme I, which depict the two enantiomers of the alcohols).Scheme I shows by way of example the synthesis of one enantiomer of theester in reaction 1, and the other possible synthetic processes forconverting the wrong enantiomer into the required enantiomer inreactions 2 to 6.

Scheme I Processes for Preparing Enantiomerically Pure Esters of theFormula I (R enantiomer or S enantiomer, Ia or Ib)

If the ester (Ia) produced in the first reaction (Scheme I) is therequired enantiomer, this is separated from the other reaction products(IIa and IV). This can take place, for example, by precipitating thealcohol (IIa) in a nonpolar solvent such as toluene and subsequentlyfiltering. The ester remains in the organic phase, and the latter can beextracted with water to remove the remaining alcohol. The unwantedalcohol enantiomer can then be either racemized after removal of IV, forexample by basic treatment, and recycled, or else converted directly tothe esters in a chemical reaction with inversion of the stereocenter,for example in a Mitsunobu reaction (see Scheme I), or in a reaction toform sulfonic anhydrides with mesylates, tosylates or brosylates andhydrolysis, or reaction with carboxylates, or converted into therequired enantiomer in a reaction to form trichloroacetimidates andsubsequent reaction with, for example, carboxylic acids or carboxylates,and subsequently esterified.

If the ester (Ia) produced in the first reaction (Scheme I) is theunwanted enantiomer, this is removed from the other reaction products(IIa and IV) for example as described above. The ester can then beeither cleaved with retention of the stereochemistry to the alcohol(IIb) (reaction 2, aminolysis or hydrolysis), racemized and recycled(reaction 3) or cleaved with racemization and recycled (reaction 4) orelse converted, after cleavage (reaction 2), in a subsequent chemicalreaction in which the stereocenter is inverted into the requiredenantiomer of the alcohol (IIa) (reaction 5). The desired enantiomer ofthe alcohol (IIa) can finally be esterified to the required ester(reaction 6).

EXAMPLES Examples 1 to 10

The enzymes used as shown in Scheme II were assayed with the followingmixture:

0.25 mmol of precursor

2.0 ml of THF or MTBE, dioxane

0.25 mmol of vinyl propionate

25 mg of enzyme

Scheme II Stereoselective esterification with vinyl esters

For the short assays, the enzymes were weighed into screw-cap tubes. Thereaction was started by adding precursor (V) and vinyl propionate (VI)in THF or MTBE/dioxane. The mixtures were incubated at room temperature(23° C.) with stirring (magnetic stirrer, 150 rpm). Samples were takenfor TLC analysis after 4 h and 24 h (TLC analysis, mobile phase ethylacetate: methanol 10:1, UV analysis). The optical rotation wasdetermined on mixtures which showed conversion in this rapid assay(optical rotation measurement: [α] ^(25° C.)/Na in ethanol, c=1).

TABLE I Optical rotations measured with various enzymes Mixture EnzymeRotation after 4 h Rotation after 24 h  1 Lipase from −0.428 −0.352Pseudomonas spec. DSM 8246  2 Novozym ® SP525 — −0.019  3 Novozym ®SP526 — +0.031  4 Subtilisin — —  5 Novozym ® SP435 −0.089 −0.335  6Chirazyme ® L1 −0.241 −0.561  7 Chirazyme ® L2 — −0.091  8 Chirazyme ®L4 −0.334 −0.314  9 Chirazyme ® L5 — +0.034 10 Chirazyme ® L6 −0.297−0.338

The activities of the enzymes mentioned in the assay with vinylpropionate and the precursor varied widely in the rapid assay(Experiments 1 to 10). Both enantiomers are formed.

Example 11

To determine the kinetics of enantiomer formation, the following largermixture was carried out with the best enzyme from Experiments 1 to 10(Chirazyme® L1):

10 mmol of precursor

80 ml of THF

10 mmol of vinyl propionate

410 mg of enzyme

The precursor (V) was introduced together with the vinyl propionate (VI)into THF. The reaction was started by adding the enzyme. Samples weretaken, and the optical rotation was measured, after incubation at roomtemperature (23° C.) for 2, 4, 6, 8, 24, 28 and 96 h. The reaction wasat a standstill after 96 h, i.e. there was no further shift between thetwo enantiomers (ester and alcohol) present in the reaction after 96 h.

TABLE II Optical rotations measured with Chirazyme ® L1 Time in hOptical rotation 2 −0.056 4 −0.091 6 −0.128 8 −0.161 24 −0.342 28 −0.35896 −0.561

Example 12

In order to determine the enantiomeric purity of the individualcomponents, a mixture was carried out as described in Example 11, andthe enantiomers (VII and VIII) were separated from one another byprecipitating the alcohol in toluene and removing the organic phase andwashing it several times with water. The enantiomeric purities of thealcohol and of the ester after cleavage with retention of thestereocenter were determined after formation of the Mosher ester (seeScheme III).

The enantiomerin purity of the two enantiomers was also determined on anHPLC column (Chiracel OD 250×4 mm, eluent 900 ml of n-hexane, 100 ml ofisopropanol, 1 ml of diethylamine, 10 ml of methanol, gradient:isocratic, flow rate: 1.0 ml/min, pressure: 28 bar, UV 254 nm, runningtime: 35 min, sample: 1 mg/5 ml of eluent).

The enantiomeric purity of the ester (VIII) was determined to be 99.1%ee by HPLC and 85% ee using the Mosher ester, and that of the alcohol tobe 66.1% ee with 40% conversion. The enzntiosectivity (E) of the enzymewas E =467.

Example 13

Conversion of the precursor with lipase from Pseudomonas spec. DSM 8246in the following mixture:

2.5 mmol of precursor

20 ml of THF or MTBE/dioxane

2.5 mmol of vinyl propionate

82 mg of lipase from P. spec. DSM 8246

The mixture was incubated with shaking (150 rpm) at room temperature(23° C). The enantiomeric purity determined by HPLC for the ester was97.5% ee and for the alcohol was 60% ee, with 38.1% conversion.

Example 14

The conversions and enantiomeric purities were determined as describedin Example 12 with the other enzymes Chirazym® L4 and L6. Theenantiomeric purity for L4 was 99.5% ee for the ester and 62.5% ee forthe alcohol, with 38.6% conversion (E=652). In order to be able tomeasure the enantiomeric purities of the two components at exactly 50%conversion, the reaction was carried out under HPLC control and thereaction was stopped at exactly 49.2% conversion. Under theseconditions, the enantiomeric purity for the enzyme L6 was 99.4% ee forthe ester and 96.1% ee for the alcohol (E=1417).

Example 15

Conversion of the precursor with lipase from Pseudomonas spec. DSM 8246in a larger mixture:

505 mmol of precursor

2500 ml of THF

505 mmol of vinyl propionate

8.3 g of lipase from P. spec. DSM 8246

The reaction was started by adding the lipase. The experiment wascarried out as described in Example 12. 99.65 g of product were isolatedafter workup. The enantiomeric purities were determined to be asfollows: ester 97% ee, alcohol >98% ee.

We claim:
 1. A process for preparing enantiomerically pure esters of theformula I

where the substituents have the following meanings: R¹ hydrogen orsubstituted or unsubstituted C₁-C₆-alkyl, C₁-C₆-alkoxy orC₁-C₆-alkanoyl, R² and R³ independently of one another hydrogen orsubstituted or unsubstituted C₁-C₆-alkyl, C₁-C₆-alkoxy, C₁-C₆-alkanoyl,C₁-C₆-alkylthio, C₁-C₆-alkylsulfinyl or C₁-C₆-alkylsulfonyl, R⁴ and R⁵R⁴≠R⁵ and independently of one another hydrogen or substituted orunsubstituted C₁-C₆-alkyl or R⁴ and R⁵ form together with the carbonatoms to which they are bonded a substituted or unsubstitutedC₃-C₆-cycloalkylidene, R⁶ substituted or unsubstituted aryl,C₁-C₂₀-alkyl, C₃-C₂₀-alkenyl, C₃-C₂₀-alkynyl, C₁-C₂₀-alkoxy-C₁-C₂₀-alkylwhich comprises converting racemic compounds of the formula II,

where the substituents R¹ to R⁵ have the abovementioned meanings, with alipase or esterase in the presence of vinyl esters of the formula III,

where R⁶ has the abovementioned meaning, and R⁷ is hydrogen or methyl,into compounds of the formula I.
 2. The process as claimed in claim 1,wherein the process is carried out in the presence of at least one inertsolvent.
 3. The process as claimed in claim 1, further comprisingremoving the alcohol of the formula II produced in the reaction.
 4. Theprocess as claimed in claim 1, further comprising cleaving theenantiomerically pure compounds of the formula 1, with retention of thestereochemistry, to compounds of the formula II.
 5. The process asclaimed in claim 1, further comprising racemizing the enantiomer of theformula II which is unwanted in each case and the recemate is returnedto the reaction.
 6. The process as claimed in any of claim 1, furthercomprising cleaving the enantiomerically pure compounds of the formulaII to compounds of the formula II and returning the cleaved compounds tothe reaction.
 7. The process as claimed in claim 1, further comprisingconverting the particular unwanted enantiomerically pure compound of theformula II in a chemical reaction with inversion of the stereocenterinto the required enantiomer.
 8. The process as claimed in claim 1,wherein the enantiomerically pure compound of the formula II areesterified with retention of the stereochemistry to compounds of theformula I.
 9. The process as claimed in claim 1, wherein a lipase oresterase of microbial origin or a porcine pancreatic lipase is used.